By Arnold Plotnick, MS, DVM, ACVIM
Toxoplasmosis is a disease caused by infection with the microscopic organism called Toxoplasma gondii (T. gondii). This is not a new disease; it was first described in 1908. Virtually all warm-blooded animals, including most pets, livestock, and people, can be infected with this organism. It is an extremely well adapted parasite and rarely causes significant disease to the individuals it infects.
How common is Toxoplasma in cats?
Toxoplasma occurs worldwide and infection in cats is similarly widespread. Many more cats are infected than show symptoms. In research studies, as many as one half of adult cats have antibodies to the organism in their blood indicating that they have been exposed to the infection at some time. Infection rates are higher in free-roaming and stray cats. In contrast, infection is uncommon in pet cats that do little or no hunting, and are fed primarily or exclusively commercial cat foods.
How is Toxoplasma transmitted?
Cats are usually infected by ingesting the organism present in the tissues (meat) of another infected animal (an ‘intermediate host’), usually a mouse, bird, or other small animalFor indoor cats, the most likely source is uncooked meat scraps. The Toxoplasma organism replicates first locally in the intestinal tract of the cat, and is often contained there. The replication in the intestinal tract results in shedding of oocysts in the feces. The oocysts represent a hardy form of the organism that can survive in the external environment for many months or even years. Other animals can become infected by ingesting these oocysts, but only if large numbers are ingested will disease result.
In some cats, particularly if their immune defenses are compromised, the Toxoplasma organisms can invade beyond the intestine and spread into various organs of the body. There, they may cause enough damage to cause signs of disease or may become dormant in a tissue cyst (not the same as the oocyst form). Such tissue cysts can be infective if the infected tissue is eaten by another animal.
How do people get Toxoplasmosis?
While cats are usually infected by preying on infected rodents (or more rarely by ingestion of oocysts from the environment), humans are most commonly infected through food. Sheep, cattle and pigs grazing on contaminated pastures, or fed oocyst-contaminated food, can also develop the encysted form of the organism in body tissues. If infected meat is not adequately cooked, or poor hygiene precautions are adopted during handling of uncooked meat, humans can become infected. Ingestion of oocysts from infected cats, for example during gardening in contaminated soil, is a less common source of human infection.
What disease does Toxoplasma cause in cats?
Although Toxoplasma is a relatively common infection, it usually causes no disease in infected cats. However, if the cat’s immune system is not working properly, Toxoplasma may continue to replicate, spread and cause damage to tissues. When this happens a variety of different clinical signs can develop including ocular (eye) disease, respiratory disease, diarrhea, liver disease and neurological signs. Such disease may be acute (rapid in onset) or more chronic with periods of illness interspersed with periods of some recovery. It is important to remember that Toxoplasma is a rare cause of disease in cats.
How can you diagnose and treat Toxoplasmosis?
Toxoplasmosis is difficult to diagnose in cats because the signs can be so variable. Blood tests are available that will demonstrate, by the presence of antibodies to the organism, whether a cat has been exposed to the organism. But these tests do not necessarily mean that Toxoplasma is the cause of any disease since most exposed cats do not develop disease. When Toxoplasmosis is suspected in a cat, it is usually treated with a course of an appropriate antibiotic.
How important is Toxoplasma in people?
Around 30% of adults in the U.S. and Europe have antibodies to Toxoplasma, which means they have been exposed to the parasite. As with infection in cats, the vast majority of people infected with this organism experience no clinical disease at all, or possibly just mild and transient ‘flu’-like signs. However, there are also some individuals where significant disease does occur and two situations are particularly important. If a pregnant woman acquires Toxoplasma infection during her pregnancy, the infection may be transmitted to the fetus, and sometimes causes severe damage. Again, this is only a risk if the woman acquires the infection during her pregnancy. A woman who has previously been exposed to the organism carries no risk of transmission to a fetus if she subsequently becomes pregnant.Ideally, women who have frequent contact with cats should be serologically tested for prior exposure to Toxoplasma gondii before becoming pregnant, because if they are already seropositive, they are not at risk for acquiring a primary infection during pregnancy.Statistics show that of the 4 million babies born in the United States each year, only 1200 suffer any effects from toxoplasmosis.Most of these babies only have mild problems, such as a rash or an eye infection.Nonetheless, the parasite can cause grave damage to a growing fetus.In rare instances, babies have developed mental retardation and anemia.
The other segment of the population at high risk for the disease is persons with compromised immune system. People with AIDS, people receiving chemotherapy, diabetics, alcoholics, and people receiving drugs to prevent rejection of a transplanted organ are at higher risk for contracting toxoplasmosis.Usually, these people have been exposed to the Toxoplasmaparasite earlier in life, and their weakened immune system is allowing the parasite to multiply unchecked.
How can human infection be avoided?
Although cats are essential to complete the life-cycle of T. gondii, numerous surveys have shown that people who own cats are not themselves at a higher risk of acquiring infection. There are several reasons for this:
- Many pet cats will never be exposed to Toxoplasma and therefore cannot pass infection on to humans.
- Even if a cat does become infected with Toxoplasma, it will only shed the oocysts (eggs) in its feces for a short period (approximately 10 days) after initial exposure. Following this there is no further significant oocyst shedding and therefore again no further risk to humans. Oocysts become infectious after 1 or 2 days. Since cats are very fastidious and are unlikely to leave feces on their fur for two days, it is unlikely that humans become infected from direct contact with cats themselves.
- Although humans can be infected through exposure to, and ingestion of oocysts in the environment, a more common source of infection appears to be infected meat.
Following a few sensible environmental and meat hygiene measures can greatly reduce the risk of human infection:
- Cook all meat thoroughly – at least 160 -180 F (70 –82 C) throughout. When cooking, avoid tasting meat before it is fully cooked. Be aware that microwaving meat is not guaranteed to killToxoplasma.
- Avoid eating unpasteurized dairy products
- Wash hands, utensils (cutting boards, knives) and surfaces (sinks, counters) carefully after handling raw meat.
- Wash all vegetables carefully, especially those grown in backyard gardens.
- Wear gloves when gardening in soil potentially contaminated by cat feces.
- Empty cat litter trays daily, dispose of litter carefully, and disinfect with boiling water. If this is done every day, even if a cat is excreting oocysts, they will not have become infectious (because it takes at least 24 hours for oocysts that are passed in the feces to “ripen” and become infectious) by the time the litter is changed.
- Discourage pet cats from hunting, and avoid feeding them raw or undercooked meat.
- Cover any children’s sand boxes to prevent cats using them as a litter tray.
- Boil water from ponds and streams when camping/hiking.